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13. Intrusion Detection


13.a. AIDE (Advanced Intrusion Detection Environment)

AIDE is a Host-Based Intrusion Detection System (HIDS), a free alternative to Tripwire (if you already know Tripwire you should have no difficulties learning the configuration file for AIDE). HIDS are used to detect changes to important system configuration files and binaries, generally by making a unique cryptographic hash for the files to be checked and storing it in a secure place. On a regular basis (such as once a day), the stored "known-good" hash is compared to the one generated from the current copy of each file, to determine if that file has changed. HIDS are a great way to detect disallowed changes to your system, but they take a little work to implement properly and make good use of.

The configuration file is based on regular expressions, macros and rules for files and directories. We have the following macros:

Macro Description Syntax
ifdef If definded @@ifdef "name"
ifndef If not defined @@ifndef "name"
define Define a variable @@define "name" "value"
undef Undefine a variable @@undef "name"
ifhost if "hostname" @@ifhost "hostname"
ifnhost if not "hostname" @@ifnhost "hostname"
endif Endif must be used after any of the above macros except define and undef @@endif

These macros become very handy if you have more than one Gentoo box and want to use AIDE on all of them. But not all machines run the same services or even have the same users.

Next we have sets of flags to check for on files and directories. These are a combination of permissions, file properties and cryptographic hashes (i.e. checksums).

Flag Description
p permissions
i inode
n number of links
u user
g group
s size
b block count
m mtime
a atime
c ctime
S check for growing size
md5 md5 checksum
sha1 sha1 checksum
rmd160 rmd160 checksum
tiger tiger checksum
R p+i+n+u+g+s+m+c+md5
L p+i+n+u+g
E Empty group
> Growing logfile p+u+g+i+n+S

And if AIDE is compiled with mhash support it supports a few other features:

Flag Description
haval haval checksum
gost gost checksum
crc32 crc32 checksum

Now you can create you own rules based on the above flags by combining them like this:

Code Listing 1.1: Create a ruleset for AIDE


The last thing we need to create our own configuration file is to see how to add a rule to a file or directory. To enter a rule, combine the file or directory name and the rule. AIDE will add all files recursively unless you specify an alternate rule.

Flag Description
! Don't add this file or directory.
= Add this directory, but not recursively.

So lets watch a full blown example:

Code Listing 1.2: /etc/aide/aide.conf

@@ifndef TOPDIR
@@define TOPDIR /

@@ifndef AIDEDIR
@@define AIDEDIR /etc/aide

@@ifhost smbserv
@@define smbactive

# The location of the database to be read.

# The location of the database to be written.


# Rule definition

@@{TOPDIR} Norm
@@ifdef smbactive
=@@{TOPDIR}home Norm

In the above example we specify with some macros where the topdir starts and where the AIDE directory is. AIDE checks the /etc/aide/aide.db file when checking for file integrity. But when updating or creating a new file it stores the information in /etc/aide/ This is done so it won't automatically overwrite the old db file. The option report_URL is not yet implemented, but the author's intention was that it should be able to e-mail or maybe even execute scripts.

The AIDE ebuild now comes with a working default configuration file, a helper script and a crontab script. The helper script does a number of tasks for you and provides an interface that is a little more script friendly. To see all available options, try aide --help. To get started, all that needs to be done is aide -i and the crontab script should detect the database and send mails as appropriate every day. We recommend that you review the /etc/aide/aide.conf file and ensure that the configuration accurately reflects what is in place on the machine.

Note: Depending on your CPU, disk access speed, and the flags you have set on files, this can take some time.

Note: Remember to set an alias so you get roots mail. Otherwise you will never know what AIDE reports.

Now there is some risk inherent with storing the db files locally, since the attacker will (if they know that AIDE is installed) most certainly try to alter the db file, update the db file or modify /usr/bin/aide. So you should create a CD or other media and put on it a copy of the .db file and the AIDE binaries.

One can find information at the AIDE project page.

13.b. Snort

Snort is a Network Intrusion Detection System (NIDS). To install and configure it use the following examples.

Code Listing 2.1: /etc/conf.d/snort

SNORT_OPTS="-q -D -u snort -d -l $LOGDIR -h $NETWORK -c $SNORT_CONF"

Copy /etc/snort/snort.conf.distrib to /etc/snort/snort.conf.

Code Listing 2.2: /etc/snort/snort.conf

~# cd /etc/snort && cp snort.conf.distrib snort.conf

You might need to comment out the blacklist and whitelist entries if no lists are created.

More information is at the Snort web site.

13.c. Detecting malware with chkrootkit

HIDS like AIDE are a great way to detect changes to your system, but it never hurts to have another line of defence. chkrootkit is a utility that scans common system files for the presence of rootkits--software designed to hide an intruder's actions and allow him to retain his access--and scans your system for likely traces of key loggers and other "malware". While chkrootkit (and alternatives like rkhunter) are useful tools, both for system maintenance and for tracking an intruder after an attack has occurred, they cannot guarantee your system is secure.

The best way to use chkrootkit to detect an intrusion is to run it routinely from cron. To start, emerge app-forensics/chkrootkit. chkrootkit can be run from the command line by the command of the same name, or from cron with an entry such as this:

Code Listing 3.1: Schedule chkrootkit as a cronjob

0 3 * * * /usr/sbin/chkrootkit

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Page updated April 10, 2014

Summary: Detect intruders.

Kim Nielsen

John P. Davis

Eric R. Stockbridge

Carl Anderson

Jorge Paulo

Sven Vermeulen

Benny Chuang

Sune Jeppesen

Tiemo Kieft

Zack Gilburd

Dan Margolis

Joshua Saddler

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